Power amplifiers are divided into class A, class AB, class B, class C, Class D and so on, by the difference of operating point. In class B amplifiers, bias current is zero when there is no signal (when no signal is inputted), and thus discontinuity occurs at a zero-cross point of alternating signal outputted when there is a signal (when a signal is inputted), causing crossover distortion. Accordingly, when no signal is inputted, a proper bias current is made to flow to reduce this distortion. Such current flowing when no signal is inputted is called an idling current; an amplifier of this type is called a class AB amplifier. Consequently, the operating point of class AB amplifier is the point where the load line slightly rises.
In class AB amplifiers, in order to implement low current consumption, there is usually used a “class AB push-pull type” in which the upper half portion (positive half period) of an alternating signal and the lower half portion (negative half period) are operated by separate transistors. In the class AB push-pull type, the upper half portion and lower half portion are driven by a rear-stage output transistor (driver) connected to a preceding-stage differential amplifier circuit (pre-driver) in push-pull form, whereby an output signal is produced (for example, refer to Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open No. 11-308057